This invention relates to the field of biohydrometallurgy. In particular, it relates to biocatalyzed leaching of precious metals, such as gold and silver, from their ores.
Development of cost-effective techniques for extraction of gold and silver from their ores has been a goal of metallurgists for hundreds of years. In recent years, incorporation of the environmental costs into the total cost of the products of gold and silver miners has encouraged a search for environmentally acceptable options.
Since its invention in 1899 by Charles Merrill of Homestake Mining Company, cyanidation has been the process of choice for extraction of gold and silver from oxidized ores. The cyanidation process is not without its limitations, however. A primary limitation is the toxicity of the leaching agent, the cyanide ion. The average fatal dose of hydrogen cyanide (HCN) for humans is 50 to 60 milligrams (McKee, J. E., & Wolf, H. W. Water Quality Criteria, 3-A, California State Water Resources Control Board, 1963). Concentrations of 0.10 to 0.15 milligrams per liter of HCN are lethal to trout.
Another limitation of the cyanidation process is that it is effective only if the ore to be leached is in an oxidized state. Typically, non-oxidized (e.g., sulfide) ores (especially those with a relatively high carbon content) are oxidized at elevated temperatures and pressures in large autoclaves, i e., "roasted", prior to cyanide leaching (McQuiston, Jr., F. W., & Shoemaker, R. S., Gold and Silver Cyanidation Plant Practice, Vol. II, Baltimore: Port City Press, 1980). As near-surface, oxidized ore reserves are mined out, however, increasing reliance must be placed on deeper, "refractory" sulfide ores.
During the last decade, heap leach processes for cost-effective bio-oxidation of pyritic and arsenopyritic sulfides in gold and silver ores have been developed to the point of commercial application (Torma, A. E., Biotechnology: A Comprehensive Treatise in 8 Volumes, Deerfield Beach, Fla.: Verlag Chemie, 1981). Recent improvements in the art are disclosed in U.S. Pat. Nos. 4,822,413, 4,987,081, 5,076,927, 5,127,942 and 5,246,486 which disclosures are incorporated by reference herein as if fully set forth (Pooley et al, U.S. Pat. No. 4,822,413, Apr. 18, 1989; Hackl et al, U.S. Pat. No. 4,987,081, Jan. 22, 1991; Hunter, U.S. Pat. No. 5,076,927 Dec. 31, 1991; Brierly et al, U.S. Pat. No. 5,127,942, Jul. 7, 1992). A great variety of precious metal extraction processes have also been developed (Gupta, C. K., & Mukherjee, T. K., Hydrometallurgy in Extraction Processes, Vol. I, Boston: CRC Press, 1990). Precious metal extraction processes are disclosed in U.S. Pat. Nos. 4,778,519 and 4,902,345 and in UK Patent No. 8,622,561, which disclosures are incorporated by reference herein as if fully set forth (Pesic, U.S. Pat. No. 4,778,519, Oct. 18, 1988; Ball et al, U.S. Pat. No. 4,902,345, Feb. 20, 1990; UK Patent No. 2,180,829, Apr. 8, 1997). The low economic cost of cyanidation however, has ensured its proliferation.
State-of-the-art precious metal heap leach practice varies with the nature of the ore. Bio-oxidation process steps may include ore crushing, acid pretreatment, inoculation with appropriate sulfide-oxidizing bacteria, addition of nutrients, recirculating the biolixiviant and cooling the heap (for 3 to 8 days), and allowing the heap to "rest" (for 3 to 8 days). Precious metal extraction process steps may include washing the heap for an extended period (e.g., 14 days) to remove residual acidity or iron content, breaking the heap apart in order to agglomerate it with cement and/or lime to make a new heap, leaching it with an alkaline cyanide or thiosulfate solution for 30 to 40 days, and recovery of gold and silver from the leach solution by adsorption on activated carbon or zinc dust precipitation.
The cyanide extraction reaction proceeds in two stages as follows (Gupta, C. K., & Mukherjee, T. K., Hydrometallurgy in Extraction Processes, Vol. I, Boston: CRC Press, 1990): EQU 2Au+4CN.sup.- +O.sub.2 +2H.sub.2 O.fwdarw.2Au(CN).sub.2.sup.- +2OH.sup.- +H.sub.2 O.sub.2 2Au+4CN.sup.- +H.sub.2 O.sub.2 .fwdarw.2Au(CN).sub.2.sup.- +2OH
The cyanide leach solution typically contains 0.25 kg of NaCN per 1,000 kg of solution (about130 mg CN.sup.- /l) and is maintained at about pH 10.5.
In that cyanidation is an open air (aerobic) process, it cannot operate in the winter when freezing of leach solutions would occur (unless the leach solution can be heated) (McQuiston, Jr., F. W., & Shoemaker, R. S., Gold and Silver Cyanidation Plant Practice, vol. II, Baltimore: Port City Press, 1980). Excess cyanide lixiviant must be oxidized (biologically or chemically, e.g., with chlorine) and, if chlorinated, dechlorinated prior to release to the environment.
A significant amount of work in the field of bio-oxidation and metals extraction has been accomplished by a variety of investigators. Tomizuka and Yagisawa describe a two-step process for leaching of uranium and oxidation of lead sulfide with recovery of metals by means of microbial sulfate reduction. Torma (1981) reviews bioleaching processes (Tomizuka, N., & Yagisawa, M. in Metallurgical Applications of Bacterial Leaching and Related Microbiological Phenomena, (eds.) Murr, L. E., Torma, A. E., & Brierley, J. A., New York: Academic Press, 1978). Livesey-Goldblatt describes a process for gold recovery from arsenopyrite/pyrite ore by bacterial leaching and cyanidation (Livesay-Goldblatt, E. 89-96, 1985. Fundamental and Applied Biohydrometallurgy. Proc. 6th International Symposium on Biohydrometallurgy, Vancouver, B.C. 89-96, 1986). Hackl et al. describe development of the BIOTANKLEACH process for leaching pyritic materials from gold and silver ore (Hackl, R. P., Wright, F., & Bruynesteyn, A., Proceedings of the Third Annual General Meeting of Biominet, Aug. 20-21, 71-90, 1986). The results of bench-scale and pilot-scale evaluations are presented. Marchant and Lawrence list considerations in the design of commercial bio-oxidation plants (Marchant, P. B., & Lawrence, R. W., Proceedings of the Third Annual General Meeting of Biominet, Aug. 20-21, 39-51, 1986). The benefits of using the BacTech moderately thermophilic cultures in bio-oxidation processes are discussed by Budden and Spencer (Budden, J. R., & Spencer, P. A., FEMS Microbiology Reviews, 11, 191-196, 1993). Chapman et al. (1993) describe a modular mobile bioleach pilot plant for bio-oxidation of a refractory gold-bearing high-arsenic sulfide concentrate.
Thermophilic versus mesophilic bioleaching process performance is evaluated by Duarte et al. (Duarte, J. C., Estrada, P. C., Pereira, P. C., & Beaumont, H. P., FEMS Microbiology Reviews, 11, 97-102, 1993). Two years of BIOX bio-oxidation pilot plant data are analyzed by Hansford and Miller (Hansford, G. S., & Miller, D. M., FEMS Microbiology Reviews,11, 175-182, 1993). Hoffman et al. (1993) describe the design of a reactor bioleach process for refractory gold treatment. Liu et al. present an evaluation of the effects of process variables on pyrite/arsenopyrite oxidation and gold extraction (Liu, X., Petersson, S., & Sandstrom, A., FEMS Microbiology Reviews, 11, 207-214, 1993). Maturana et al. describe an integrated biological process for treatment of a complex gold ore(Maturana, H., Lagos, U., Flores, V., Gaeta, M., Cornejo, L., & Wiertz, J. V., FEMS Microbiology Reviews, 11, 215-220, 1993). Mineral sulfide oxidation by enrichment cultures of a novel thermoacidophilic bacteria are described by Norris and Owen (1993). Rate controls on the bio-oxidation of heaps of pyritic material imposed by bacterial upper temperature limits are described by Pantelis and Ritchie (Pantelis, G., & Ritchie, A.I.M., FEMS Microbiology Reviews, 11, 183-190, 1993).
Bio-oxidation bacteria have been characterized in detail. Brierley and Brierley characterize a chemoautotrophic and thermophilic (70.degree. C.) microorganism isolated from an acid hot spring (Brierley, C. L., & Brierley, J. A., Canadian J. Microbiology, 19, 183-188, 1973). De Rosa et al. characterize the extremely thermophilic (85.degree. C.), acidophilic (pH 1.0) bacteria Sulfolobus acidocaldarius (De Rosa, M., Gambacorta, A., & Bullock, J. D., J. General Microbiology, 86, 156-164, 1975). Torma et al. present a kinetic analysis of the growth of Thiobacillus ferrooxidans in a synthetic medium (Torma, A. E., Biotechnology: A Comprehensive Treatise in 8 Volumes, Deerfield Beach, Fla.: Verlag Chemie, 1981).
Henley presents data characterizing the solubility of gold in chloride solutions in the temperature range 300.degree.-500.degree. C. (Henley, R. W., Chemical Geology, 11, 73-87, 1973). Puddephatt reviews the chemistry of gold (Puddephatt, R. J., The Chemistry of Gold, New York: Elsevier Scientific Pub. Co., 1978). The chemistry of thiourea and thiosulfate extraction of gold and silver is discussed by Block-Bolten et al. (Block-Bolten, A., Daita, M. S., Torma, A. E., & Steensma, R., Recycle and secondary Recovery of Metals, Pennsylvania: Metallurgical Society, Inc., 1985).
Hydrogen sulfide gas (H.sub.2 S) may be produced biologically by the action of sulfate-reducing bacteria. Hydrogen sulfide in aqueous solution is called hydrosulfuric acid. Hydrosulfuric acid ionizes as follows. EQU H.sub.2 S&lt;=&gt;H.sup.+ +HS.sup.- EQU HS.sup.- &lt;=&gt;H.sup.+ +S.sup.-2
The HS.sup.- ion is called the bisulfide ion. The S.sup.-2 ion is called the sulfide ion. At a partial pressure of one atmosphere, the solubility of H.sub.2 S in water is about 3,400 mg/l (0.1 molar) at 25.degree. C. and about 3,800 mg/l (0.11 molar) at 20.degree. C., of which about half exists as HS.sup.- ion at pH 7.
Neutral bisulfide/sulfide solutions dissolve gold and silver by creating very stable bisulfide and sulfide complexes (Barnes, H. L. (ed.), Geochemistry of Hydrothermal Ore Deposits, 2nd ed., New York: John Wiley & Sons, 1979). Weissberg has estimated that, at 25.degree. C. in neutral bisulfide solutions, gold has a solubility of 40mg/l in0.17M HS.sup.- and 125 mg/l in 0.32M HS.sup.- at a pressure of 1,000 atmospheres (Weissberg, B. C., Economic Geology, 65, 551-556, 1970). Seward found that, in the region of alkaline pH, at temperatures below 200.degree. C., "the effect of pressure on the solubility of gold is relatively minor." (Seward, T. M., Geochimica et Cosmochimica Acta, 37, 379-399, 1973) He also noted that "a pronounced solubility maximum occurs in the region of pH about 7 . . . " and that "higher hydrogen fugacities . . . considerably depress the solubility of gold." Ag.sub.2 S solubility exceeds 10 mg/l in near-neutral solutions at 20.degree. C. and increases to nearly 40mg/l at 100.degree. C. (Barnes, H. L. (ed.), Geochemistry of Hydrothermal Ore Deposits, 2nd ed., New York: John Wiley & Sons, 1979) .
In conventional bio-oxidation of gold and silver ores, a waste sulfuric acid stream is produced. Means for bioprocessing of this waste stream are disclosed in U.S. Pat. No. 5,076,927, which disclosure is incorporated by reference herein as if fully set forth.